SEOmoz have been seeing extreme volatility in EC2 spot pricing - could this have been related to pre-release testing of Silk?
http://devblog.seomoz.org/2011/09/amazon-ec2-spot-request-vo...
http://devblog.seomoz.org/2011/09/amazon-ec2-spot-request-vo...
(The idea that the spot price is allowed to exceed the regular price is also curious.)